1. Field of the Invention
The present invention relates to a device and a method for buffer-storing a multiplicity of wafer-type workpieces vertically one above another and individually without the workpieces touching one another. The device comprises a frame, at least two transport elements which circulate in a vertical direction and which are provided, at uniform intervals, with a multiplicity of bearing areas for the horizontal mounting of workpieces, a loading position and a stationary removal device.
2. Background Art
Various products in modern industry require semifinished products in the form of very precisely processed wafer-type workpieces. These are, for example, annular wafers composed of glass or aluminum as substrates for the production of magnetic mass storage devices (hard disks) for computers, optical glasses, highly level reference surfaces for optical purposes (so-called “Flats”), multicrystalline semiconductor wafers for the production of photovoltaic cells, etc. Particularly stringent requirements are made of monocrystalline semiconductor wafers as starting material for functional components appertaining to electronics, microelectronics and microelectromechanics.
Semiconductor wafers are produced by means of a multiplicity of successive process steps that can generally be classified into the following groups:
(a) production of a usually monocrystalline semiconductor rod;
(b) slicing of the rod into individual wafers;
(c) mechanical processing;
(d) chemical processing;
(e) chemomechanical processing;
(f) if appropriate additional production of layer structures.
What are advantageous and therefore frequently used in the production of semiconductor wafers are particularly those processes from groups (b) to (f) in which a plurality of semiconductor wafers are processed simultaneously in one device. This form of processing is called group processing or a batch process. Batch processes from group (b) include, for example, multi-wire slicing (MWS), from group (c) lapping or grinding with planetary kinematics, from group (d) etching or chemical cleaning in a bath, and from group (e) double-sided polishing (DSP) using silica sol, for example.
What is common to all the batch processes mentioned is that, at the end of a processing cycle, a plurality of processed semiconductor wafers are obtained simultaneously or within a short time for further processing. Therefore, in contrast to single-wafer or continuous processing methods, batch processes are distinguished by a temporally nonuniform material flow.
Prior to further processing, it is necessary to clean the semiconductor wafers in order to remove the lapping agent adhering to the semiconductor wafers after MWS or lapping processing or the polishing agent adhering after DSP or the grinding slurry adhering after grinding processing. Preferably, the cleaning is effected directly after the preceding lapping, grinding or polishing processing, as long as the semiconductor wafers are still wet, since lapping or polishing agent or grinding slurry, once it has dried in, adheres very strongly to the surface or even damages the latter.
The prior art discloses a multiplicity of cleaning methods which are in each case tailored to the type of contamination present and the degree of cleanness to be achieved. These methods are subdivided into batch cleaning methods, in which a plurality of semiconductor wafers are cleaned simultaneously in one cleaning device, and single-wafer cleaning methods, in which the semiconductor wafers are cleaned individually and successively, either cyclically sequentially or continuously in continuous cleaning methods.
U.S. Pat. No. 6,423,149 BA describes, for example, a cleaning device, comprising a plurality of pairs of mutually opposite cylindrical sponges which rotate about their longitudinal axes and between which semiconductor wafers are led through individually successively in a continuous passage movement by means of conveyor belts and, by means of contact and relative movement of sponge and semiconductor wafer surfaces with respect to one another and the supply of a cleaning liquid, both sides of the semiconductor wafers are cleaned simultaneously (single-wafer cleaning method with continuous passage).
Cleaning methods of this type have proved to have particularly high performance. However, these methods always operate with cyclic or continuous passage of individual semiconductor wafers, since each surface of each semiconductor wafer fed for cleaning has to be swept over completely by a cleaning tool. This is not possible in batch cleaning methods.
Consequently, in many cases there is the problem that semiconductor wafers obtained in bunches after processing in a batch process have to be separated and fed for cleaning successively cyclically or with continuous passage.
Furthermore, for reasons of economic viability, it is undesirable that, for example, the installation operator who unloads the semiconductor wafers after processing by a batch process and feeds them manually for subsequent cleaning adapts his unloading pace to the cleaning throughput, since this leads to waiting times, increased outlay on operating personnel, reduced material throughput and quality losses. Quality losses can arise in the case of DSP, for example, if polishing agent adhering to the surface of the semiconductor wafer dries thereon.
In addition, it is necessary in many cases for the semiconductor wafers to be fed for cleaning in direct proximity to the removal from the batch process, in order to avoid a change in the properties of the semiconductor wafer, for example as a result of oxidation or incipient etching caused by residues of a chemically active processing liquid of the batch process that have remained on the semiconductor wafer, and in order, in the case of manual transport, for example, to increase the ergonomics and work safety and in order to minimize the risk of damage to the semiconductor wafer as a result of careless handling, the risk of interchange of front and rear sides of the semiconductor wafer as a result of unintentional rotation, or the risk of interchange of the order of the semiconductor wafers removed from the batch process on the transport path.
Finally, it is often desirable for each batch processing device to be assigned exactly one cleaning device, for example in order to avoid mixing up semiconductor wafers from different batch processing devices, and for the cleaning device to be able to be embodied in a very compact and space-saving fashion, in order that it can be retrofitted to an existing batch processing device, for example, without having to interrupt the batch processing operation for a long time or even having to change the disposition of the batch processing device.
The prior art discloses devices which can pick up a multiplicity of wafer-type workpieces individually successively or simultaneously and release them again. These devices are designated as “buffer stores”, “buffers” or the like. For semiconductor wafers, these temporary storage or stacking devices are designated as “wafer stockers”.
JP2006-032528A describes, for example, a device comprising four pairs of continuous (closed) chains, of which the two chains in each pair are connected to one another by sprockets. Two first chain pairs are led by means of two respective deflection roller pairs in the form of a closed inner ring. The two remaining chain pairs are led by means of four respective deflection roller pairs in each case concentrically with respect to one of the first two chain pairs (inner rings) in a likewise closed outer ring around the first chain pairs, such that two pairs of inner and outer chain pairs are respectively opposite one another. In this case, all the chains in the first half of each chain pair run in a common first plane and the chains in the other half of each chain pair run in a second plane, which is arranged parallel to the first plane. Furthermore, all the sprockets of all the chain pairs also run parallel to one another.
Planar workpieces, for example glass plates for Flat-Panel-Displays, can be placed onto the sprockets, such that each workpiece is supported by four sprockets. As a result of the synchronous driving of all the deflection rollers, the sprocket network can be adjusted in height, such that a multiplicity of workpieces can be stacked one above another without touching one another. In this case, the workpieces are led in a horizontal position by transport rollers to a loading position and, as a result of synchronous driving of all the deflection rollers, are brought into contact with the sprockets situated below the workpiece and are conveyed upward by one distance between sprockets by the latter. The next workpiece can then be fed as described to the subsequent sprockets, etc., with the result that finally a stack of all the workpieces thus supplied arises. Unloading takes place analogously to loading by means of the workpieces being progressively moved down to the unloading position, which is identical to the loading position, with the workpiece being transported out of the device by means of transport rollers. Loading and unloading proceed in the same direction and at opposite sides of the two concentric chain ring pair arrangements.
On account of the construction of the buffer store from JP2006-032528A, the workpiece introduced into the stack last is the first to be removed again from the stack (“last in first out” principle, LIFO). Thus, the order in which the workpieces are supplied during loading is reversed during unloading. Moreover, only exactly one workpiece can be either loaded or unloaded respectively at a point in time. Therefore, it is not possible, in particular, to pick up a plurality of semiconductor wafers within a short time from a preceding batch processing process and to simultaneously remove them individually again successively in time with the subsequent cleaning and in the order of inclusion in the stack (“first in first out” principle, FIFO).
Moreover, the buffer store described in JP2006-032528A also does not permit fast direct loading by hand, for example by an installation operator who removes the semiconductor wafers after the end of the preceding batch processing process rapidly, successively and in a manner maintaining the order from the processing device, since loading takes place by means of the transport rollers through between the sprockets. This movement can take place only comparatively slowly since the semiconductor wafer has to be moved for this purpose by a distance corresponding at least to its diameter, and it therefore constitutes the speed-determining step. The workpieces would additionally have to be placed onto the moving transport rollers, which would inevitably lead to undesirable friction between transport rollers and workpiece.
Moreover, the lateral loading necessitates an additional space requirement of such a device, as a result of which it cannot simply be attached to the processing device that supplies the semiconductor wafers, or it cannot easily be retrofitted if necessary without in general having to shift and rearrange the processing device. In many cases it is even completely impossible to shift the processing device, since, in modern manufacturing sequences, in general a multiplicity of different processing devices are installed at small distances from one another or in predefined building grids, the arrangement of which can no longer be changed subsequently without disturbing the entire processing sequence in the long term or changing it permanently.
Thus, it would be desirable to provide a device and a method which make it possible, after batch processing has taken place, to buffer-store a multiplicity of semiconductor wafers in the true order and in a space-saving manner and to actually feed them during the feeding of semiconductor wafers to the buffer store or directly after the feeding of the last semiconductor wafer, to subsequent single-wafer processing, for example continuous cleaning, with the cyclic timing required therefor and with their original order being maintained. Moreover, it would be desirable for the device to be capable of being manually loaded rapidly and by means of a simple movement, to solve the problems associated with the prior art.